Process for producing salt having a reduced calcium sulfate content

ABSTRACT

A process for preparing from brine (an aqueous solution of salt) containing appreciable quantities of dissolved calcium sulfate either (1) high purity salt (sodium chloride) having the usual cubic crystalline form or (2) high purity dendritic salt, either form of salt being characterized by an exceptionally low calcium sulfate content. The process for the preparation of the cubic crystalline form of sodium chloride (hereinafter also referred to as &#39;&#39;&#39;&#39;salt&#39;&#39;&#39;&#39;) is carried out by either (1) a &#39;&#39;&#39;&#39;feed and bleed&#39;&#39;&#39;&#39; procedure comprising admixing an alkali metal polyphosphate with said brine to increase the supersaturation of calcium sulfate therein, feeding the brine containing this additive into an evaporating and crystallizing chamber, evaporating the brine at an elevated temperature and reduced pressure to cause crystallization of pure salt and concomitantly bleeding brine from the chamber, the rate of feed of the brine to the chamber and the rate of bleed of brine from the chamber being such as to maintain the calcium sulfate in the dissolved state and prevent its precipitation with the salt, or by (2) subjecting brine to solar evaporation to concentrate same to the so-called &#39;&#39;&#39;&#39;salt point&#39;&#39;&#39;&#39;, i.e., that point at which salt will crystallize from the brine if the brine is subjected to slightly more evaporation, adding to said salt-point brine an alkali metal polyphosphate to increase the supersaturation of calcium sulfate therein, feeding the brine containing the polyphosphate to a primary evaporating pond and subjecting the brine to solar evaporation to cause the continuous crystallization of salt therefrom and concomitantly bleeding brine from the primary evaporating pond to a secondary evaporating pond, the rate of feed of brine to the primary evaporating pond and the rate of bleed of brine therefrom being such as to maintain the calcium sulfate in the dissolved state in the primary evaporating pond and prevent its precipitation with the salt. The process for the preparation of dendritic salt is the same feed and bleed process as utilized for the preparation of cubic crystalline salt, except that an alkali metal ferrocyanide is added to the brine in addition to an alkali metal polyphosphate.

United States Patent [191 Fiedelman 5] Dec. 9, 1975 PROCESS FORPRODUCING SALT HAVING A REDUCED CALCIUM SULFATE [57] ABSTRACT CONTENT Aprocess for preparing from brine (an aqueous solu- [75] Inventor: HowardW. Fiedelman, Woodstock, tion of salt) containing appreciable quantitiesof disllsolved calcium sulfate either 1) high purity salt (sodiumchloride) having the usual cubic crystalline form [73] Asslgnee' g z ggfif Products or (2) high purity dendritic salt, either form of saltbeing characterized by an exceptionally low calcium 1 Filed! 1975sulfate content. The process for the preparation of the cubiccrystalline form of sodium chloride (hereinafter [211 App! 554649 alsoreferred to as salt) is carried out by either (1) Related US.Application Data a feed and bleed procedure comprising admixing an [62]Division of $61. No. 480,615, June 19, 1974. alkali metal p yp p WithSaid brine to increase the supersaturation of calcium sulfate therein,feeding 52 U.S. Cl. 23/296; 23/300; 23/303; the brine containing thiSadditive into an evaporating 423/184; 423/499; 159/17 R; 203/7 andcrystallizing chamber, evaporating the brine at an [51] I (1 C011) 3/06;C011) 3/16; 3011) 9/02 elevated temperature and reduced pressure tocause 58 Field of Search 23/272 AH, 296, 297, 29s, crystallization of psalt and concomitantly bleeding [56] References Cited UNITED STATESPATENTS 1,950,459 3/1934 Seifert 23/303 2,108,783 2/1938 Smith....23/272 AI-I 2,906,599 9/1959 Roland.. 23/272 AH 3,148,023 9/1964 Ploss23/300 3,155,458 1 H1964 Fiedelman. 23/272 AH 3,308,062 3/1967 Gunther23/272 AH 3,358,740 12/1967 Akirnoto.... 23/303 3,682,601 8/1972Fedosoff 23/272 AH FOREIGN PATENTS OR APPLICATIONS 224,032 9/1959Australia 23/303 609,576 1 H1960 Canada 23/303 663,618 5/1963 Canada23/303 609,576 1 1/1960 Canada 23/303 199,607 6/1923 United Kingdom23/272 AH 711,663 6/1965 Canada 23/303 627,446 9/1961 Canada 848,3289/1960 United Kingdom 23/303 OTHER PUBLICATIONS SWD/95 1st Inter Symp.on Water Desal, Wash, DC. Oct. 3-9, 1965, pp. 1 to 15.

Primary Examiner-Norman Yudkoff Assistant ExaminerS. J. Emery Attorney,Agent, or FirmJack Axelrood ID BNINE WELL I! brine from the chamber, therate of feed of the brine to the chamber and the rate of bleed of brinefrom the chamber being such as to maintain the calcium sulfate in thedissolved state and prevent its precipitation with the salt, or by (2)subjecting brine to solar evaporation to concentrate same to theso-called salt point, i.e., that point at which salt will crystallizefrom the brine if the brine is subjected to slightly more evaporation,adding to said salt-point brine an alkali metal polyphosphate toincrease the supersaturation of calcium sulfate therein, feeding thebrine containing the polyphosphate to a primary evaporating pond andsubjecting the brine to solar evaporation to cause the continuouscrystallization of salt therefrom andconcomitantly bleeding brine fromthe primary evaporating pond to a secondary evaporating pond, the rateof feed of brine to the primary evaporating pond and the rate of bleedof brine therefrom being such as to maintain the calcium sulfate in thedissolved state in the primary evaporating pond and prevent itsprecipitation with the salt.

The process for the preparation of dendritic salt is the same feed andbleed process as utilized for the preparation of cubic crystalline salt,except that an alkali metal ferrocyanide is added to the brine inaddition to an alkali metal polyphosphate.

3 Claims, 1 Drawing Figure FEED I TANK l2 FEED BRINE I20 In SamuM NEXA-mznwnosvnfli SOLUTION 1 AND samuM rsn-mcnmni Ir PAN] PANZ nncuvc-miDENDRITIC SALT PAN .5 PAN O RETURN earn: 40

" 58 ORvsn newt/m smmurnm SALT FILTER- IZI Z6 .3! RETURN same BLEED samesm SLL'RRY TANK FlLTER omen HIGH PURITV SALT US. atent Dec.9, 1975PROCESS FOR PRODUCING SALT HAVING A REDUCED CALCIUM SULFATE CONTENT Thisis a division of Ser. No. 480,615, filed June 19, 1974.

BACKGROUND OF THE lNVENTlON of calcium sulfate, expressed as calcium.Such high calcium sulfate content presents serious problems to certainsegments of the food industry, especially in the processing of certainvegetables, in pickling brines containing phosphates and in thepreparation of salted butter. Some segments of the chemical industrywhich use salt in their operations are also adversely affected by thepresence of excessive amounts of calcium sulfate.

1n the conventional production of salt by the solar evaporation methodas much as 1500 parts per million of calcium sulfate expressed ascalcium is contained in the salt.

2. Description of Prior Art It is known to the art to produce salt bythe evaporation of brine in vacuum pans at elevated temperatures andreduced pressures to cause the crystallization of salt. The brine isusually provided from brine wells or by the dissolution of crude rocksalt. In either case, the principal containment of the brine is calciumsulfate, which is usually present in the range of from about 0.5 toabout 5.5 grams per liter. Other contaminants are calcium chloride,magnesium chloride or sulfate, and calcium bicarbonate. Further, it iscustomary to maintain solid calcium sulfate in the brine in theevaporators, approximately to 200 grams per liter, for the purpose ofpreventing scaling of the evaporator heating tubes by the precipitationthereon of the calcium sulfate present in the brine, with the attendantloss of efficiency. As the brine is concentrated in the evaporator, andsalt crystallizes therefrom, a portion of the calcium sulfate presentalso precipitates together with the salt as an internal inclusion withinthe salt crystals, and also as an external adherent. Thus the result isthat it is not possible to remove this internal contaminant, nor is itpossible to remove all of the external contaminant by simple washing.

It is also known to the art to prepare dendritic salt in a vacuum panoperation by the addition to brine of crystal-modifying agents includingmetal oxalates and ferrocyanides. The method in common use requires thatthe brine be treated to free it of its calcium and magnesium content,and also that the agitation of the brine in the evaporators be limitedto insure production of the desired crystal form. The problems ofpreparing pure salt from brine and of reducing the calcium sulfatecontamination thereof, and also the production of dendritic salt havebeen the subject ofa number of patents.

Thus, U.S. Pat. No. 2,108,783 discloses the use of sodiumhexametaphosphate as a sequestering agent for preventing thecrystallization of calcium and magnesium during an ordinary brineevaporation process. The quantity of sodium hexametaphosphate neccessaryis at least'about 8 times the quantity of calcium present in the brine.Thus, for brine containing 2.5 to 5.5 grams per liter of calciumsulfate, it would be necessary to add 20 to 44 grams per liter of sodiumhexametaphosphate.

U.S. Pat. No. 2,902,418 relates to incorporating into the dissolvingwater conducted to a salt deposit a combination of (1) a water solublecarbonate and (2) a mo lecularly dehydrated alkali metal phosphatehaving a defined Na O to P 0 ratio.

U.S. Pat. No. 2,906,559 relates to preparing sodium chloride brines oflow calcium sulfate content by dissolving crude rock salt in water inthe presence of a polyphosphate. The statement is made in this patentthat if I add the polyphosphate compounds of this invention to sodiumchloride brine after it has been prepared by dissolving rock salt orother crude salt in water, there is virtually no effect whatever inreduction of the total amount of impurities present in the brine.

U.S. Pat. No. 2,906,600 relates to the use ofa combination of apolyphosphoric acid compound and a compound containing alkaline earthmetal ions as an additive to the water used to dissolve crude salt toprepare a brine low in calcium sulfate content.

U.S. Pat. No. 2,977,189 discloses that pure brine is produced by addingto sodium chloride containing calcium sulfate as an impurity, or to thewater employed to dissolve said sodium chloride, or to both sodiumchloride and the dissolving water a small amount of( 1) an alkali metalcarbonate and (2) an alkali metal phosphate selected from the groupconsisting of alkali metal pyrophosphates, hexametaphosphates, andtripolyphosphates.

U.S. Pat. No. 3,148,023 produces tetrakaidecahedra crystals of sodiumchloride by dissolving in a substantially pure saturated aqueous sodiumchloride solution a combination of sodium hexametaphosphate and apolyvalent metal ion (preferably aluminum).

U.S. Pat. No. 2,433,601 discloses the preparation of a brine low incalcium sulfate content from crude brine by dissolving the crude sodiumchloride in water containing small amounts of sodium carbonate ortrisodium phosphate. In this process, it is important that the pH of thewater be at least about 7 so that an insoluble precipitate of calciumcarbonate or calcium phosphate is formed which can be removed.

U.S. Pat. No. 3,025,013 teaches a process whereby an acidic compoundsupplying the phosphate ion (PO- is injected at intervals into a streamof water used to dissolve crude rock salt and a base is also added tosaid water to provide a water pH of at least about 7.

US. Pat. No. 3,155,458 relates to the use of starch phosphate in a feedand bleed process for the production of sodium chloride having a lowcalcium sulfate content. The use of starch phosphate in the process isstated to be operable only on a substantially bicarbonate-free brine.U.S. Pat. No. 2,642,335, relates to the production of dendritic salt,i.e., salt having a crystalline structure in the form of hollow cubeswith corners having branchedspikes thereon in three dimensions. Thisspecial form of salt is obtained by the addition to brine of acrystal-modifying substance including metal oxalates, metalferrocyanides and metal ferricyanides, together with the evaporation ofthe brine under conditions of reduced agitation. To prepare a highpurity dendritic salt by this method, i.e., one which contains less thanabout parts per million of calcium sulfate expressed as calcium,requires the use of a treated brine. A treated brine is one from whichessentially all of the calcium and most of the magnesium conent areremoved as precipitates by the addition of chemical agents such as sodaash (Na CO and caustic soda (NaOI-I). If untreated brine is used in thepreparation of dendritic salt by the method of U.S. Pat. No. 2,642,335,the resultant product contains the usual level of up to about 1500 partsper million of calcium sulfate expressed as calcium.

Thus, each of the foregoing prior art processes for the preparation ofbrine, or salt having a reduced calcium sulfate content offers certaindisadvantages. For example: U.S. Pat. No. 2,108,783 requires the use oflarge quantities of sodium hexametaphosphate, i.e., about 8 times thequantity of calcium sulfate present in the brine. U.S. Pat. No.2,902,418 teaches the necessity of adding a combination of a carbonateand a particularly defined phosphate to the water used to dissolve saltfrom a salt deposit for the purpose of preventing the dissolution ofcalcium sulfate. This patent does not teach a method for maintainingcalcium sulfate in solution in a brine in which it is already present,as for example, in brine from a brine well.

U.S. Pat. No. 2,906,599 teaches the use of a polyphosphate addition tothe water used to dissolve crude rock salt to reduce the dissolution ofcalcium sulfate. Again, this fails to solve the problem of providingsubstantially calcium sulfate-free salt from an existing brinecontaining this impurity.

U.S. Pat. No. 2,906,600 is directed to the same concept as U.S. Pat. No.2,906,599, i.e., reducing the dissolution of calcium sulfate whileforming a brine from crude rock salt. U.S. Pat. No. 2,977,189 is similarin effect to U.S. Pat. No. 2,902,418 in that it too teaches a method ofreducing the dissolution of calcium sulfate while forming a brine fromcrude salt. The principal difference appears to be that U.S. Pat. No.2,977,189 employs a combination of carbonate and polyphosphate, whereasU.S. Pat. No. 2,902,418 uses a combination of carbonate and a specificmolecularly dehydrated phosphate.

U.S. Pat. No. 3,148,023 addresses itself to the production of aparticular crystalline form of sodium chloride and requires the additionof a combination of sodium hexametaphosphate and a compound of apolyvalent metal to pure saturated brine. Obviously, the process of thispatent depends upon first providing a purified brine.

U.S. Pat. No. 2,433,601 teaches the removal of calcium sulfate frombrine by precipitation of calcium as the carbonate or the phosphate.This process requires adjustment of the pH of the water where it is lessthan 7, and also requires the removal and disposal of precipitates.

U.S. Pat. No. 3,205,013, as do several of the other prior art patents,uses a combination of a phosphate (acidic) and an alkaline compoundwhich is added to the water used to dissolve crude rock salt to preventthe concomitant dissolution of excessive quantities of calcium sulfate.Here again, pH adjustment to about 7 is necessary.

U.S. Pat. No. 3,155,458, which employs a starch phosphate compound in afeed and bleed process, suffers from the following defects inherent inthe use of starch phosphate:

1. Starch phosphate has a limited solubility in water (about 2%) therebymaking it difficult to prepare aqueous solutions suitable for additionto brine.

2. Starch phosphate is subject to bacterial attack and thereforedeteriorates upon standing.

3. Starch phosphate is operable only in bicarbonatefree brine.

U.S. Pat. No. 2,642,335 teaches a process for the production ofdendritic salt which requires (1) the use of treated brine to providesalt having a calcium sulfate content less than about parts per millioncalcium sulfate expressed as calcium, and also (2) reduced agitation ofthe brine in the evaporators to obtain the dendritic structure.

Accordingly, it would be desirable to provide (1) a vacuum panevaporation process for preparing from crude brine containingappreciable quantities of calcium sulfate cubic crystalline salt ordendritic salt, each having a markedly reduced calcium sulfate content,and (2) a solar evaporation process for preparing from crude brinecontaining apppeciable quantities of calcium sulfate cubic crystallinesalt having a markedly reduced calcium sulfate content. It would bedesirable to provide the foregoing without the necessity of employingcombinations of phosphates and alkaline compounds, or without thenecessity of pH adjustment, or without the necessity of precipitationand precipitateremoving procedures, or without the necessity ofmaintaining solid calcium sulfate in' the brine in the vacuum pans, orwithout the necessity of avoiding vigorous agitation of brine or withoutemploying substances which have limited solubility in water, or limitedstability, or are otherwise limited with respect to the type of brine inwhich they function.

It is therefore an object of this invention to provide a vacuum panprocess for producing either cubic crystalline salt or dendritic salt,each having a very low calcium sulfate content, which process canutilize raw or crude brine.

It is also an object of this invention to provide a solar evaporationprocess for producing cubic crystalline salt having a veyr low calciumsulfate content.

It is another object of this invention to provide a vacuum pan processfor the production of pure cubic crys-- talline salt and dendritic salthaving a very low calcium sulfate content which does not require themaintenance of solid calcium sulfate in the brine in the crystallizingchambers.

It is a further object of this invention to provide a vacuum pan processfor the production of pure cubic crystalline salt and dendritic salthaving a very low calcium sulfate content which does not require the useof large quantities of any extraneous compound or combinations ofcompounds.

It is a still further object of this invention to provide a process forthe production of pure cubic crystalline salt and dendritic salt havinga very low calcium sulfate content in a salt crystallizing operationwhich process obviates the formation of calcium precipitates and thenecessity of removing same.

It is another object of this invention to provide a vacuum pan processfor the production of pure cubic crystalline salt and dendritic salthaving a very low calcium sulfate content from crude brine containingcalcium sulfate as an impurity which process does not require pHadjustment of the brine or the removal of bicarbon- I Broadly, oneaspect of the present invention comprises a process for preparing a highpurity cubic crystalline sodium chloride having a reduced calciumsulfate content which comprises continuously feeding raw sodium chloridebrine containing dissolved calcium sulfate into an evaporating andcrystallizing chamber, concentrating and evaporating said brine at anelevated temperature and at a reduced pressure in the presence of atleast about parts per million on a saturated brine basis of an alkalimetal polyphosphate to produce a crystalline sodium chloride,concomitantly bleeding brine from said evaporating chamber, the rate offeed of raw brine to said evaporating chamber and the rate of bleed ofbrine therefrom being adjusted to maintain calcium sulfate in solution,and concomitantly withdrawing solid sodium chloride crystals from saidevaporating chamber.

Another aspect of this invention comprises a process for preparingdendritic salt of high purity and exceptionally low calcium sulfatecontent which comprises continuously feeding raw sodium chloride brinecontaining dissolved calcium sulfate into an evaporating andcrystallizing chamber, concentrating and evaporating said brine at anelevated temperature and at a reduced pressure in the presence of atleast about 5 parts per million on a saturated brine basis of an alkalimetal polyphosphate, adding to said brine an alkali metal ferrocyanideat a rate sufficient to produce a crystalline dendritic salt having abulk density not exceeding about 59 pounds per cubic foot and containingless than about 13 parts per million of said alkali metal ferrocyanide,concomitantly bleeding brine from said evaporating chamber, the rate offeed of raw brine to said evaporating chamber and the rate of bleed ofbrine therefrom being adjusted to maintain calcium sulfate in solution,and concomitantly withdrawing solid sodium chloride crystals from saidevaporating chamber.

The pure cubic crystalline salt or dendritic salt crystallized by thepresent feed and bleed vacuum pan process may be filtered, washed anddried by any convenient means.

A further aspect of the present invention comprises subjecting brine tosolar evaporation to concentrate same to the salt point, adding to saidsalt point brine at least about 5 parts per million on a saturated brinebasis of an alkali metal polyphosphate to increase the supersaturationof calcium sulfate therein, feeding said salt point brine containingsaid polyphosphate to a primary evaporating pond and subjecting saidbrine therein to solar evaporation to cause the continuouscrystallization of cubic crystalline salt therefrom and concomitantlybleeding brine from the primary evaporating pond to a secondaryevaporating pond, the rate of feed of salt point brine containingpolyphosphate to the primary evaporating pond and the rate of bleed ofbrine therefrom to said secondary pond being such as to maintain thecalcium sulfate present in the brine in the primary evaporating pond inthe dissolved state and thereby prevent its precipitation with the saltin said primary pond. The pure crystallized salt deposited in theprimary pond is harvested in the usual manner.

As compared with the standard vacuum pan method of producing high puritysalt which requires pre-treatment of the brine to chemically removeimpurities (cal-- cium and magnesium salts) followed by separation ofbrine from the precipitated solids, the present feed and bleed processutilizing raw brine provides substantial savings in cost of chemicals,labor, and equipment.

The terms very low calcium sulfate content" and reduced calcium sulfatecontent are here taken to mean a calcium sulfate content ranging as lowas about 25 parts per million and up to about parts per million,expressed as Ca.

Calcium sulfate is generally inversely soluble in brine with increasingtemperature. This accounts for its precipitation together with sodiumchloride in evaporators (vacuum pans) which are maintained at elevatedtemperatures. As previously stated,'there have been many methodsprovided by the prior art to either remove calcium sulfate from brineprior to the entry of the brine into an evaporating chamber, or toinhibit its dissolution in the water used to dissolve salt to makebrine. Surprisingly, it was found that an alkali metal polyphosphate,when present in very small quantities, has the unexpected property ofgreatly increasing the supersaturation of calcium sulfate in saturatedbrine. This unusual phenomenon makes it possible to add an alkali metalpolyphosphate directly to raw brine and evaporate and crystallize salttherefrom in a solar evaporation process or at elevated temperatures ina vacuum pan operation before the solubility of calcium sulfate isexceeded and coprecipitates with the salt. As the evaporation processcontinues and the concentration of calcium sulfate in the brineincreases, the calcium sulfateenriched brine is bled from the evaporatoror from the solar evaporating pond before calcium sulfate canprecipitate. Concomitantly with the bleeding of the calciumsulfate-enriched brine from the evaporator or the pond, fresh brine towhich an alkali metal polyphosphate has been added is fed into theevaporator or the pond to maintain the continuity of the process.

The term alkali metal polyphosphate is here taken to include sodiumhexametaphosphate, potassium hexametaphosphate, sodium tripolyphosphate,potassium tripolyphosphate, tetrasodium pyrophosphate, andtetrapotassium pyrophosphate. The preferred alkali metal polyphosphateis sodium hexametaphosphate (hereinafter SHMP), and reference will behad to this compound in the present specification as illustrative of analkali metal polyphosphate. The unique function of the alkali metalpolyphosphates in the present invention is illustrated by the fact thatorthophosphates are not effective to increase the supersaturation ofcalcium sulfate is saturated brine, as is demonstrated hereinafter byExample 2.

It has been found that for brine at a temperature of about F. andcontaining about 5 grams per liter of calcium sulfate, the addition ofas little as 5 parts per million of SHMP on a saturated brine basis(0.0005%) is sufficient to increase the apparent solubility of calciumsulfate to 9.3 grams per liter, a solubility increase of 1.8 times.lncreasing the SHMP concentration to 100 parts per million at thistemperature increases the apparent solubility of calcium sulfate toabout 10.5 grams per liter, or a concentration increase (orconcentration factor) approximately 2.1 times the originalconcentration. As is illustrated by the examples, a concentration factoras high as 2.5 is attained by coordinating the parameters of brinetemperature and SHMP concentration in the initial and feed brines. Itwas, of course, totally unexpected that so minute a quantity of SHMPwould have so great an effect on the apparent solubility of calciumsulfate. The concentration of SHMP may range from about 5 to about 500parts per million, and preferably from about 5 to about 100 parts permillion on a saturated brine basis. Inasmuch as the concentration ofsodium chloride in saturated brine is about 25 to about 26%, on a saltbasis the concentration of SHMP would range from about to about 2000parts per million.

The dendritic salt produced by the present feed and bleed processutilizing both an alkali metal polyphosphate and an alkali metalferrocyanide produces salt having a hollow cubical structure withthree-dimensional branched spikes on the corners thereof. This specialform of salt prevents close packing and results in a bulk density thatis much lower than that of conventional salt having a cubic crystallineform. For example, the dendritic salt prepared by the process of thisinvention has a bulk density ranging from about 53 to about 59 poundsper cubic foot, or from about 0.85 gm./cc. to about 0.93 gm./cc. By wayof contrast, the bulk density of cubic crystalline salt is in the rangeof about 72 pounds per cubic foot. Advantages inherent in dendritic saltover cubic crystalline are superior non-caking characteristics and afaster rate of dissolution in water and superior blending properties.

Although any of the alkali metal ferrocyanides are operable in thisprocess, sodium ferrocyanide is preferred. The rate of addition ofalkali metal ferrocyanide is adjusted during operation of the presentfeed and bleed process to achieve maximum salt crystal modification tothe dendritic form and to obtain dendritic salt having an alkali metalferrocyanide content less than aboutl3 parts per million. The FederalFood and Drug Administration permits up to l3 parts per million ofsodium ferrocyanide in salt for human consumption.

The present process may be carried out in either a single evaporator orin a multiple effect evaporator systern. The terms evaporator,evaporating and crystallizing chamber, and vacuum pan or pan are usedinterchangeably herein. In a four-pan multiple effect vacuum pan system,for example, the first vacuum pan generally is operated under conditionsof the highest temperature and pressure. The second pan has the secondhighest temperature and pressure, the third pan has the third highesttemperature and pressure and the fourth pan has the lowest temperatureand pressure. The cooler fourth and third pans are usually selected forcarrying out the present feed and bleed process, although it should beunderstood that the process is not limited to these pans and can becarried out in the hotter pans. The temperatures generally employed inmultiple effect evaporators vary from a high of about 260F. in aforced-circulation pan and about 225F. in a Calandria pan to a low ofabout 90 to 100F. in a Calandria pan and about 125F. in aforced-circulation type evaporator.

It should be understood that the above temperatures and pressures aretypical for a quadruple effect system. It is possible to use additionalpans in which case the temperatures and pressures would vary somewhatfrom the aforementioned values.

In the conventional type vacuum pan system, the deposition of calciumsulfate gradually builds up during the course of operation andaccumulates on the heating surfaces thereby diminishing the efficiencyof operation and necessitating periodic cleaning. it is customary to addsolid calcium sulfate to the brine in the vacuum pans to prevent itsdeposition with the aforementioned disadvantages. By the practice of thepresent feed and bleed process in the presence of small quantities ofSHMP, it is not necessary to add solid calcium sulfate as the entireproblem of calcium sulfate deposition is obviated. Further, the presentprocess is operable in the presence of calcium bicarbonate which is acontaminant usually encountered in raw brine. The removal of bicarbonateis required for the successful operation of certain salt crystallizingprocesses, particularly that of US. Pat. No. 3,l55,453 previouslydiscussed.

in one preferred embodiment of the present invention relating to avacuum pan operation, the rate of feed of fresh brine to the evaporatorand bleed of calcium sulfate-enriched brine therefrom is concatenatedsuch that calcium sulfate is maintained in solution and not permitted toprecipitate or crystallize together with sodium chloride. Inasmuch asthe concentration in saturated brine of calcium sulfate is increased bya factor of about 2 by the addition of SHMP, calcium sulfateenrichedbrine must be bled from the vacuum pan prior to the time when theconcentration of calcium sulfate reaches this level. Consequently,periodic analyses are made of the calcium sulfate content of the brinein the evaporator so that the calcium sulfate-enriched brine is bledwhen the level of calcium sulfate reaches a concentration factor ofabout 1.8 times to provide a rea sonable margin of safety. On anoperational basis, it has been found that from about 40 to about of thefeed brine is bled from the evaporator.

For the preparation of pure cubic crystalline salt by the presentinvention using solar evaporation of brine, periodic analysis are madeof the calcium sulfate content of the brine in the primary evaporatingpan so that the calcium sulfate-enriched brine is bled from the primaryevaporator when the level of calcium sulfate reaches a concentrationfactor of about 1.5 times to provide a reasonable margin of safetyagainst its precipitation with the salt. Calcium sulfate-enriched brinefrom the primary evaporating pond is bled to a secondary crystallizingpond where evaporation is permitted to continue to the normal bitternpoint, i.e, that point at which magnesium sulfate begins to precipitate.As is readily apparent, salt crystallized in the secondary pond containsa greater calcium content than that crystallized in the primary pond.

For a clearer understanding of the vacuum pan process of the presentinvention, reference is now made to the drawing illustrating the presentfeed and bleed process.

The drawing is a flow diagram depicting a four-pan salt productionprocess employing the present feed and bleed procedure. it is seen thatthere are four vacuum pans or evaporators identified as Pan 1, Pan 2,Pan 3, and Pan 4, respectively. Pan 4, is here referred to as the feedand bleed evaporator, and Fans 1, 2, and 3 as conventional evaporators.Raw brine (feed brine) from brine well 10 is conducted via line 12 tofeed tank 16 from which it is distributed to Pans l, 2, and 3 via lines12a, 12b, and 12c, respectively. Feed brine is also conducted throughline 14 to mixing point 18. An aqueous solution of sodiumhexametaphosphate, and of sodium ferrocyanide if dendritic salt is beingproduced,

is contained in tank 20, and is injected through line 22 into said feedbrine at mixing point 18 by means of proportioning pump 24. The rate ofsodium hexametaphosphate addition is such as to provide a concentrationon a saturated brine basis of from about 5 to 500 parts per million. Therate of sodium ferrocyanide addition, if used, is adjusted to providedendritic salt containing no more than 13 parts per million of sodiumferrocyanide. The raw brine, now containing sodium hexametaphosphate(and sodium ferrocyanide, it

used), continues through line 22 to Pan 4. The temper ature in this feedand bleed evaporator is maintained at about 130F. if it is a forcedcirculation type, or from about 100 to about 120F. if it is a so calledCalandria type, under reduced pressures ranging from a low of about 1.5for a Calandria pan to a high of about 3.5 inches of mercury (absolute)for a forced circulation type.

As water is evaporated from Pan 4, the concentration of salt exceeds itssolubility in the brin and pure salt crystallizes therefrom to form asalt slurry. This slurry is drained periodically (or continuously, ifdesired) via line 26 into salt slurry tank 28. The slurry is thenconducted via line 29 to a suitable filter-dryer 30 where the solid saltis separated from the slurry by conventional filtration means, washedwith water and then dried to provide a pure sodium chloride having nomore than about 25 to 100 parts per million of calcium and magnesiumcontent expressed as calcium.

The concentration of calcium sulfate in solution in Pan 4 increases andbuilds up as water is evaporated and as salt is crystallized. It isimportant to the successful operation of the present process and to theproduction of pure salt that the calcium sulfate be maintained in thedissolved state and not be permitted to precipitate in Pan 4 and therebycontaminate the salt. As previously mentioned, sodium hexametaphosphatehas the unexpected effect of increasing the supersaturation of calciumsulfate in brine, thereby permitting more water to be evaporated fromthe brine before calcium sulfate begins to precipitate. To determinethat point at which calcium sulfate will precipitate requires periodicmonitoring (analyses) of the brine in this pan to insure that theconcentration of calcium sulfate does not exceed about 1.8 to 2.0 timesits concentration in the original raw brine. When its concentrationapproaches a concentration factor of about 1.8, the calciumsulfateenriched brine is bled from Pan 4 via line 32 to salt slurry tank34 where it is commingled with salt slurry from Pans 1, 2, and 3. Ifdesired, this bleed brine may also be fed directly to Pans l, 2, or 3.When the brine is so bled, fresh feed brine admixed with SHMP isintroduced into Pan 4 as before. The rate of brine bleeding from Pan 4and the rate of feed of fresh brine containing SHMP into Pan 4 areconcatenated so that the quantity of calcium sulfate dissolved in thebrine is always maintained at a concentration below its precipitationpoint.

Pans 1, 2, and 3 are operated in the conventional manner. That is, theyare arranged in a cascade manner with respect to the heat source. Thus,steam for heating purposes is fed to a steam jacket surrounding Pan 1.Vapor from Pan 1 is passed to a steam jacket surrounding Pan 2, vaporfrom Pan 2 is passed to a steam jacket surrounding Pan 3, and vapor fromPan 3 is passed to the steam jacket surrounding Pan 4. If theevaporating pans are of the forced circulation type, then operatingconditions therein may be in the temperature range of from about240-260F. at a pressure of 38 to 53 inches of mercury (absolute) in Pan1, from about 200 to 220F. at a pressure of 17.5 to 26 inches of mercury(absolute) in Pan 2, and from about 170 to about 190F. at a pressure of9 to 14 inches of mercury (absolute) in Pan 3. If the evaporating pansare of the Calandria type, the operating conditions therein may be inthe temperature range of from about 200 to about 220F. at a pressure of17.5 to 26 inches of mercury (absolute) for Pan 1, from about 175 toabout 190F.

at a pressure of 10 to 14 inches of mercury (absolute) in Pan 2, andfrom about 140 to about 160F. at a pressure of 4.5 to 7.0 inches ofmercury (absolute) in Pan 3.

Salt slurry from Pans 1, 2, and 3 is withdrawn through lines 12d, 12e,and 12f, respectively, into salt slurry tank 34, from which tank theslurry is conducted to washer 36 via line 35. The salt slurry receives acountercurrent brine wash and the washed slurry is then passed throughline 37 to filter-dryer 38 where the salt is separated by filtration anddried to provide the final conventional granulated salt product. Thebrine from filter 38 is recycled to feed tank 16 through line 40. Excessbrine from washer 36 may be recycled to feed tank 16 via line 39 whichjoins line 40 atjuncture 42.

Regular cubic crystalline salt produced by the conventional vacuum pantechnique contains from about 600 to 1500 parts per million of calciumsulfate, expressed as calcium. If it were possible to make dendriticsalt from raw, untreated brine by a conventional vacuum process, thesalt would contain up to 1500 parts per million of calcium sulfateexpressed as calcium. This is to be contrasted with cubic crystallinesalt or dendritic salt produced by the present feed and bleed processutilizing raw brine in which the concentration of calcium sulfate rangesfrom about 25 to about 100 parts per million, expressed as calcium.

It should be noted that the present feed and bleed process provides forthe retention in solution of calcium sulfate and the prevention of itsprecipitation with the salt. By way of contrast, as previouslymentioned, in the conventional method for the production of salt by theaforedescribed vacuum pan process it is common practice to maintain asuspension of solid calcium sulfate in the pans to prevent scaling ofthe heating surfaces by calcium sulfate which continually precipitatesduring the evaporation process and contaminates the salt.

Although the present feed and bleed process can be carried outwith anytype of brine, even that containing considerable quantities of calciumbicarbonate, a bicarbonate removal step may be incorporated in thesystem, if desired. This is done by conducting feed brine to a separatetank where it is treated with sodium hydroxide to precipitate calciumcarbonate therefrom. The bicarbonate-free brine is then admixed withSHMP prior to entry in Pan 4, and the feed and bleed process is carriedout as described.

For a more complete understanding of the present invention, reference isnow made to the following specific examples which illustrate the highpurity salt obtained by this feed and bleed process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1 The effect of variousquantities of SHMP on the solubility of calcium sulfate in saturatedbrine was determined in the following manner:

A saturated brine, maintained at a temperature of F. was also saturatedwith respect to calcium sulfate by dissolving therein 5.5 grams ofcalcium sulfate per liter. To one portion of this brine there was added5 parts per million of SHMP. This brine was then placed in a vesselunder vacuum and constant agitation to evaporate water therefrom andthereby increase the concentration of calcium sulfate therein.Periodically, the calcium content was determined until the point wasreached where calcium sulfate precipitation occurred. This procedure wasrepeated with 12.5, 50, and 100 parts per million of SHMP respectively.

The results were as follows:

The procedure of Example 1 was repeated except that 30 parts per millionof trisodium phosphate (an orthophosphate) was substituted for SHMP. Theconcentration at which calcium sulfate precipitation occurred was 5.5grams per liter, or the same as that with no additive present. Thisdemonstrates that an orthophosphate has no effect in increasing thesupersaturation of calcium sulfate in saturated brine.

EXAMPLE 3 Salt was produced by the process of this invention in a feedand bleed evaporator operated as part of a quadruple effect set (Pans 1,2, 3 and 4) of forced-circulation vacuum pans. The feed and bleedevaporator was Pan 4, to which there was added clear raw brine from abrine well. Analysis showed that this brine contained 3.4 grams ofcalcium sulfate per liter, which is the saturation level for calciumsulfate in this particular brine which also contains 5.1 grams ofcalcium chloride per liter. Just prior to the entry of this brine intoPan 4, an aqueous solution of SHMP containing 0.335 pounds of SHMP pergallon was added at a rate to provide a SHMP concentration in the brineof 25 parts per million on a saturated brine basis.

The temperature in Pan 4 was maintained at a range of from about 125 toabout 135F. and at a reduced pressure of about 3.5 inches of mercury(absolute). Pans 1, 2, and 3 were operated in the conventional manner inwhich calcium sulfate was maintained in suspension and in which thefollowing temperatures and pressure conditions were maintained:

Pressure Inches Temperature of Mercury (Absolute) Pan 1 247F. 42.3 Pan 2208F. 20.3 Pan 3 172F. 9.4

with only 30 parts per million of calcium and magnesium combined,expressed as calcium.

EXAMPLE 4 The following experiment was conducted to evaluate theoperability of alkali metal polyphosphates in increasing thesupersaturation of calcium sulfate in saturated brine:

To a sample of saturated brine containing 5.2 grams per liter of calciumsulfate and maintained at a temperature of 134F. and reduced pressurethere was added 30 parts per million of SHMP. As evapo ration proceeded,feed brine containing 20 parts per million of SHMP was added to maintainconstant volume. Samples of filtered brine were removed periodically andanalyzed for calcium content. Evaporation was continued until themaximum calcium sulfate solubility was reached and the level of calciumsulfate in the brine started to decrease. In this way, the maximum levelof calcium sulfate solubility was determined. The results were asfollows:

Maximum G.P.L. CaSO,

Reached in Brine Initial Brine (pp Feed Brine (PP Conc. Factor Temp.

Additive F.

None Blank SHMP EXAMPLES 5-13 Maximum G.P.L. Initial Feed CaSO BrineBrine Temp. Reached Concf Ex. Additive (ppm) (ppm) F. 1n Brine FactorNone-Blank 134 5.2 5 SHMP 30 20 9.64 1.85 6 SHMP 30 20 8.04 1.55 7 SHMP60 40 134 13.05 7.50 8 SHMP 60 40 155 11.20 2.16 9 SHMP 60 40 175 10.001.92

Sodium Tripoly- 10 phosphate 30 20 134 9.25 1.78 11 30 20 155 8.30 1.6012 30 20 175 7.90 1.52

Tetrasodium Pyro- 13 phosphate 20 15 134 7.73 1.49

Maximum G.P.i.. CaSO Reached Concentration Factor EXAMPLE 14 Theprocedure of Example 3 was repeated except that dendritic salt wasproduced in a triple effect Calandria set of vacuum pans. In addition tothe aqueous solution of SHMP added to the No. 3 Pan, :1 10% solution ofsodium ferrocyanide was added at a rate which was adjusted to produceproper crystal modification (dendritic salt). The dendritic salt soproduced had a bulk density of 55 lbs/ft. (pounds per cubic foot) and aso dium ferrocyanide content of 12 parts per million. Continuedoperation produced dendritic salt having a bulk density varying from53-59 lbs/ft. and less than 13 parts per million of sodium ferrocyanide.

The following temperature and pressure conditions were maintained.

Pressure-Inches Temp. F. of Mercury Absolute Pan 1 210 21 Pan 2 170 9Pan 3 120 2.5

additive consisting of from about 5 to about 500 parts per millionsodium hexametaphosphate to crystallize sodium chloride from said brine,concomitantly bleeding brine from said evaporating chamber in an amountof from 45% to of the brine feed, concomitantly withdrawing andrecovering solid sodium chloride having a reduced calcium sulfatecontent of not more than about 30 parts per million of calcium expressedas calcium from said evaporating chamber, directing said bleed brinecontaining an enriched dissolved calcium sulfate content to a separateevaporating and crystallizing chamber, concentrating said bleed brine tocrystallize sodium chloride therefrom, and recovering the sodiumchloride.

2. The process of claim 1 wherein the brine is bled from saidevaporating chamber prior to the time when the calcium sulfate contentof said brine reaches a concentration from about one and one half toabout twice that of its concentration in the raw brine.

3. The process of claim 1 wherein the sodium hexametaphosphate ispresent in an amount of from about 5 to about parts per million.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Q Patent No.3,925,027 Dated December 9, 1975 Inventor(s) HOWARD W. FIEDELMAN It iscertified that error appears in the above-identified patent e and thatsaid Letters Patent are hereby corrected as shown below:

1. Column 1, line 34, second word "containment" should be contaminant.

. 2o Column 2, line 45 u.s. Patent No 3,025,013 s/b U.S. Patent NO.3,205,013o

. 3. Column 2, line 47 (PO should be (P5?) 0 4, Column 4, line 37 v I II II The word very 1S misspelled 'veyr 5. Column 6, line 46 The wordafter calcium sulfate shouldbe "in" instead of "is",

Bugncd and Scaled this Sixteenth D y Of March 1976 Attest: I

e RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioneroj'latems and Trademarks

1. AN IMPROVED PROCESS FOR PRODUCING CUBIC CRYSTALLINE SALT IN AMULTIPLE EFFECT EVAPORATOR SYSTEM WHICH COMPRISES CONTINUOUSLY FEEDING ARAW SODIUM CHLORIDE BRINE CONTAINING DISSOLVED CALCIUM SULFATE INTO ANEVAPORATING AND CRYSTALLIZING CHAMBER, EVAPORATING AND CONCENTRATINGSAID BRINE AT A TEMPERATURE BELOW ABOUT 220*F. AND AT REDUCED PRESSUREIN THE PRESENCE OF AN ADDITIVE CONSISTING OF FROM ABOUT 5 TO ABOUT 500PARTS PER MILLION SODIUM HEXAMETAPHOSPHATE TO CRYSTALLIZE SODIUMCHLORIDE FROM SAID BRINE, CONCOMITANTLY BLEEDING BRINE FROM SAIDEVAPORATING CHAMBER IN AN AMOUNT OF FROM 45% TO 70% OF THE BRINE FEED,CONCOMITANTLY WITHDRAWING AND RECOVERING SOLID SODIUM CHLORIDE HAVING AREDUCED CALCIUM SULFATE CONTENT OF NOT MORE THAN ABOUT 30 PARTS PERMILLION OF CALCIUM EXPRESSED AS CALCIUM FROM SAID EVAPORATING CHAMBER,DIRECTING SAID BLEED BRINE CONTAINING AN ENRICHED DISSOLVED CALCIUMSULFATE CONTENT TO A SEPARATE EVAPORATING AND CRYSTALLIZE LIZINGCHAMBER, CONCENTRACTING SAID BLEED BRINE TO CRYSTALLIZE SODIUM CHLORIDETHEREFROM, AND RECOVERING THE SODIUM CHLORIDE.
 2. The process of claim 1wherein the brine is bled from said evaporating chamber prior to thetime when the calcium sulfate content of said brine reaches aconcentration from about one and one half to about twice that of itsconcentration in the raw brine.
 3. The process of claim 1 wherein thesodium hexametaphosphate is present in an amount of from about 5 toabout 100 parts per million.